Ocular aberrations typically produce unwanted results in the form of bad eyesight. To be adequately treatable, these aberrations need to be measured and characterized. To this end, various devices, apparatuses, and methods have been developed for objectively measuring characteristics, including aberrations, of a subject's eye. The measured characteristics of the eye may be employed for planning corrective actions, including for example ocular surgery such as Laser-Assisted in situ Keratomileusis (“LASIK”), laser cataract surgery, and various other treatments.
LASIK and other eye surgeries are typically planned based on the far point refractive characteristics of the eye. So it is important that an optical measurement system ensures that the subject's eye is drawn to its farthest possible refractive state when making measurements for planning the surgery.
Furthermore, when eye surgery is being planned, it is desired to measure the eye when the pupil has a large diameter so that the optical characteristics of the eye can be measured over a large area of the eye. This not only affects the treatment planning, but is also important in disqualifying a subject from being a candidate for eye surgery if certain optical abnormalities are found in the wavefront map of the eye. Such irregularities can be an indication of keratoconus or other problems. For this reason, many equipment manufacturers disqualify a subject from eye surgery if the pupil diameter is less than some minimum threshold diameter (e.g., 5 mm).
However, there are some problems in simultaneously insuring that the subject's eye is drawn to its farthest possible refractive state when making measurements, while also maintaining the pupil open with as large of a diameter as possible.
To meet the requirement that that insure that the subject's eye should be drawn to its farthest possible refractive state when making measurements, many optical measurement systems employ an internal visible target for the subject to look at or follow while the measurements are made, and the target is intended to draw the eye to its farthest possible refractive state. If this target is too bright, then the pupil will close and become smaller. Hence, it is desired to make the target as dim as possible. This is especially the case with older people, who often have a habitually small pupil. Thus, a dimmer target can open an older person's pupil to be wider.
On the other hand, however, if the target is too dim, then for some younger people, the eye might not follow the target and reach its farthest possible refractive state. In this case, it is said that the eye is “accommodated.” This occurs because young people are capable of changing the refractive state of their eyes to focus at near or far distances, which in general is not the case for older people. It has been observed clinically that, for a younger person whose eye is capable of accommodation, increasing the brightness level of the target will increase the likelihood of ensuring that optical measurements are made with the eye in its farthest possible refractive state.
To address these conflicting requirements, many optical measurement systems employ a target whose brightness level is an attempted compromise between being too bright for older subjects, and too dim for younger subjects. However, in practice, there seems to be no single target brightness level that achieves a satisfactory compromise.